Data processing systems, in conjunction with processing data, typically are required to store large amounts of data (or records), which data can be efficiently accessed, modified, and re-stored. While several different mediums of data storage are typically provided within a data processing system (electronic memory, direct access storage devices (DASD), and magnetic tape), magnetic tape has an advantage of reduced cost for very large data storage capabilities, for example, gigabytes of data storage per tape. Tape storage is often used for back-up purposes, that is, data stored on one storage medium is reproduced on magnetic tape for safekeeping. Having a back-up data copy is mandatory for many businesses as data loss could be catastrophic to the business. Large volumes of back-up data, for example, terabytes of data, are generally stored in a tape library, using a robotic mechanism to select a tape for access to data thereon or for writing back-up data thereto. Increasing the data capacity that can be stored to tape reduces the cost of backing up data and improves library efficiency.
In the information storage industry, increases in tape data capacities have been achieved, in part, by increasing tape lengths stored within a cartridge or reel by producing thinner tape substrates. Additional increases in data capacity are made possible by various data compression techniques, for example, Improved Data Recording Capability (IDRC) by International Business Machines, Company, or Lempel-Ziv data compression by Stac Electronics, Inc. Data compression techniques can increase data density by two to five times over non-compressed data.
Advances in magnetic tape media and tape head technologies have generated further increases in data capacity by increasing both a number of tracks on a magnetic tape (by narrowing a width of each track) and the number of read/write elements on the tape head. An eighteen-track tape for data storage has been a standard for many years. More recently, an IBM 3490-E magnetic tape subsystem for 12.7 mm (1/2 inch) wide tape employs a head element with thirty-six read/write elements and has a tape capacity of 800 megabytes (increasing to two or more gigabytes with data compression). The 3490-E magnetic tape subsystem performs bi-directional linear recording (as opposed to helical scan recording).
A number of tape rewinds is reduced, and performance improved accordingly, by interleaving sets of head elements. In an interleaved head, element pairs having a read-element/write-element configuration (when viewed towards the face of the head) alternate with element pairs having an opposite write-element/read-element configuration, each pair of elements being associated with one track on the tape. When the tape travels in a first direction, the element pairs having one configuration access the corresponding tracks (such as the even numbered tracks) in a read-after-write manner. Conversely, when the tape travels in an opposite direction, those element pairs having the other configuration access the other corresponding tracks (such as the odd numbered tracks), also in a read-after-write manner. Further enhancing performance with such a large number of tracks requires an equally large number of closely spaced elements. The 3490-E tape head is such a head with a magneto-resistive transducer formed through thin film deposition techniques.
Historically, the number of tracks that could be established on the tape media has been limited by a number of corresponding elements that could be fabricated on a tape head. As a result, tape drives have been designed to process a tape using a head having fewer sets of read/write elements than a number of tracks on the tape. The tracks are divided into groups, each group containing a same number of tracks as there are read/write element pairs in the head. Accessing all the groups requires indexing the head transversely relative to the tape path, such as with a stepper motor or voice coil driven springs, into a number of discrete positions corresponding to the number of groups of tracks. A head having eight read/write pairs can accommodate a tape having twenty-four tracks if the tracks are divided into three groups of eight tracks each and the head has three index positions. A distance the head travels between index positions is minimized by interleaving the groups. For example, each of the three groups containing eight tracks (consecutively numbered 0-23) results in tracks 0, 3, 6, . . . , and 21 being in the first group and accessible by the head being indexed to position zero. Similarly, tracks 1, 4, 7, . . . , and 22 are in the second group, accessible in index position one, while tracks 2, 5, 8, . . . , and 23 are in the third group, accessible in index position 2.
Despite advances in data capacity, still further increases are desired, such as would be possible with a 12.7 mm wide tape having 64 or even 128 tracks. However, even when a head is indexed, there is a practical limit to the ability of a multi-track head to accurately and reliably record data to and read data from a tape having such a large number of very narrow tracks. Problems can be caused by track mis-registrations, such as tape edge variations, environmental thermal expansion and contraction and inaccuracies in the path the tape follows in a drive, as well as by inaccuracies in the formatting of tracks on the tape itself. Even a minute "wobble" in the tape can result in significant signal degradation, such as crosstalk and dropout, if a 12.7 mm tape has 128 tracks, each with a width of about eighty microns.
Using fixed heads, as in the past, made it easier to deal with track mis-registrations, etc. Now, however, not only are the tracks narrower, but the head is servo-controlled. Thus the head must appear fixed in terms of a home position, while providing excellent accuracy while servoing across track groups. Hence, very accurate head-to-tape path adjustments are required. These adjustments include track registration, penetration, yaw and azimuth. If any of these fine adjustments falls out of tolerance, or if the head itself fails, then the tape drive itself needs to be accessed to either make the adjustment, or to replace the head or field replaceable head unit (FRU) and re-adjust after installing the FRU. Such adjustment and/or replacement is costly and timely having to be performed on site or requiring shipping the entire tape device. In the case where the tape path and guides are fully contained within the tape cartridge, alignment demands may not be achievable.
Accordingly it is desired to provide a method and apparatus for accurately aligning a servo-controlled head to a tape path in a field replaceable unit (FRU), independent of the actual tape device.